In order to undergo proper cell division, eukaryotic cells must faithfully replicate their genome and then correctly segregate their chromosomes into two daughter cells. This process of cell division, also called the cell cycle, is a step-wise process that is governed by checkpoints to ensure genomic integrity. Upon completion of DNA replication (S-phase), cells enter a growth phase (G2-phase) prior to proceeding into mitosis for chromosome segregation (M-phase). A key regulator of mitosis is the kinase Cdk1 (as called Cdc2) (Nurse, P. (1990) Universal control mechanism regulating onset of M-phase. Nature 344, 503-508). Activation of Cdk1 results in the onset of mitosis, and its subsequent inactivation initiates the exit from mitosis. Cdk1 is activated by the binding of Cyclin A or Cyclin B. Both Cyclin A-Cdk1 and Cyclin B-Cdk1 complexes function to initiate mitosis (Lindqvist, A., et. Al. (2009) The decision to enter mitosis: feedback and redundancy in the mitotic entry network. The Journal of cell biology 185, 193-202). The degradation of Cyclin B triggers the inactivation of Cdk1, resulting in the mitotic exit and entry into a growth (G1) phase prior to beginning a new round of the cell cycle (Glotzer, M., et al. (1991) Cyclin is degraded by the ubiquitin pathway. Nature 349, 132-138).
In addition to Cyclins, Cdk1 is also regulated by Wee1, an atypical tyrosine kinase that phosphorylates Cdk1 on tyrosine 15 (Y15) and inactivates Cdk1 (McGowan, C. H., et al. (1993) Human Wee1 kinase inhibits cell division by phosphorylating p34cdc2 exclusively on Tyr15. The EMBO journal 12, 75-85; Parker, L. L., et al. (1992) Inactivation of the p34cdc2-cyclin B complex by the human WEE1 tyrosine kinase. Science 257, 1955-1957). Wee1 is a critical negative regulator of Cdk1 and functions at the G2-M phase checkpoint to ensure that DNA replication has been completed and the genome is not damaged prior to entering mitosis (O'Connell, et al. (1997) Chk1 is a wee1 kinase in the G2 DNA damage checkpoint inhibiting cdc2 by Y15 phosphorylation. The EMBO journal 16, 545-554). Loss of Wee1 can result in premature entry into mitosis, resulting in mitotic catastrophe and cell death (Stumpff, J., et al. (2004) Drosophila Wee1 kinase regulates Cdk1 and mitotic entry during embryogenesis. Curr Biol 14, 2143-2148). Furthermore, many cancers are defective in their G1-phase checkpoints and are reliant on G2-M phase checkpoints (Sancar, A., et al. (2004) Molecular mechanisms of mammalian DNA repair and the DNA damage checkpoints. Annual review of biochemistry 73, 39-85). Indeed, loss of expression of Wee1 has been shown to lead to the abrogation of the G2-M phase checkpoint and sensitize tumor cells to DNA damage, especially tumors that have lost their G1-phase checkpoint due to a deficiency in the p53 protein (Wang, Y., et al. (2004) Knockdown of Chk1, Wee1 and Myt1 by RNA interference abrogates G2 checkpoint and induces apoptosis. Cancer biology & therapy 3, 305-313).
Inhibitors of Wee1 have the potential to selectively cause lethality in cancerous cells that are defective in other cell cycle checkpoints, while sparing normal tissues that can activate other cell cycle checkpoints. Thus, small molecule inhibitors of Wee1 would be beneficial for therapeutic intervention in cancer and other cell proliferative disorders.